Electric double layer capacitors are required to have a high withstand voltage. The withstand voltage of such capacitors depends on an electrolytic solution. Therefore, it is necessary to obtain an electrolytic solution having a high withstand voltage.
When an electrolytic solution is conventionally selected, electrolytic solutions are prepared by combining a large number of assumed solutes and solvents, and whether the electrolytic solutions have a target withstand voltage is determined by measuring the withstand voltage of the electrolytic solutions.
However, there are infinite numbers of solutes and solvents that can be actually assumed. Therefore, in order to extract the target electrolytic solution therefrom, it is necessary to prepare electrolytic solutions with appropriately selected combinations of solutes and solvents and to repeat again and again a trial and error procedure of measuring the withstand voltage of the electrolytic solutions. Thus, with the above-described procedure, a very long time is required to attain the target electrolytic solution and, therefore, a very long time is required to develop an electric double layer capacitor.